The striking features of BRCA2 null cells are their hypersensitivity to DNA damaging agents and amplification of centrosomes. It has been reported that the centrosomes were amplified in a 65% of the BRCA2-/- MEFs. These data suggest that BRCA2 plays an important role in the regulation of centrosome duplication. However, how BRCA2 mediates the regulation of centrosome duplication remains undefined. One reasonable hypothesis is that BRCA2 directly interacts with some centrosomal proteins. Using a C-terminal fragment of BRCA2 as bait, we identified a novel BRCA2 binding protein, referred to as B2BP1 (BRCA2 binding protein 1). We have already demonstrated that BRCA2 can interact with B2BP1 both in vitro and in vivo. Sequence analysis of B2BP1 indicated that it is a novel gene, with a weak homology with various coiled-coil proteins, including two known centrosome proteins, pericentrin and ninein. Extensive immunofluorescence studies demonstrated that B2BP1 is a centrosomal protein, which strongly indicates a role for B2BP1 in the regulation of centrosome duplication. This predicted function of B2BP1 is well in accordance with the known functions of BRCA2. Therefore, we propose to characterize B2BP1 protein, localization, interaction and co-localization with BRCA2, and interaction with tubulins. We also propose to elucidate the role of the B2BP1-BRCA2 interaction in the normal function of centrosome (e.g. microtubule nucleation, spindle formation) and the regulation of centrosome duplication. Our finding that BRCA2 directly interacts with a centrosomal protein reveals an important link between BRCA2 and the regulation of centrosome duplication. Further characterization of B2BP1 should provide important insights into the mechanism of BRCA2 mutation-mediated tumorigenesis.